1. Field of the Invention
The present invention relates to a gas ballast system for a multi-stage positive displacement pump a first stage of which includes an inlet valve and an outlet valve and is connected with following stages via an intermediate vacuum chamber.
2. Description of the Prior Art
At present, multi-stage positive displacement pumps are increasingly used as forevacuum pumps for high vacuum pumps such as, e.g., turbomolecular pumps. In order for a high vacuum pump to reach its full capacity, the associated forevacumm pump should attain pressure of about 1-5 mbar. As a rule, vapor, e.g., water vapor is admixed to the pumped-out gas. During the compression stage, the vapor can condense in the forevacuum pump and, thus, is not conveyed further. To prevent the condensation of the vapor in the fore-vacuum pumps, formed, e.g., as rotary vane pumps, gas ballast systems are used. To this end, the gas from the atmosphere is admitted into the pump chamber. Specific design means in the rotary vane pumps prevents the admitted gas from reaching the suction chamber so that the admitted gas has little influence on the achievable end pressure. In this way, the condensing of vapor in the rotary vane pumps is prevented. When two-stage rotary vane pumps are used, the gas ballast is admitted only into the second stage. With the two-stage rotary vane pumps, the vapor can condense in the first stage. However, because oil is conveyed into the following stage with each revolution, condensate will also be conveyed into the second stage, together with the oil, where it is again evaporated and is expelled, together with the gas ballast air.
However, because in the rotary vane pumps, the pump chambers are sealed with oil, in many cases, they cannot be used as forevacuum pumps for turbomolecular pumps. By further development of the turbomolecular pumps, which have a last stage in a form of, e.g., a molecular pump such as a Holweck pump, the operational region of such pump combination has been expanded toward higher pressures. Thereby, it became possible to reduce the costs of obtaining of a forevacuum with respect to the pump size and the end pressure. In particular, it became possible to substitute dry pumps, e.g., diaphragm pumps, for oil sealed vacuum pumps. This is particular important in applications in which oil-free vacuum is required. The use of a diaphragm pump as a forevacuum pump particularly makes sense when the turbomolecular pump is formed as a magnetically supported turbomolecular pump. In this case, in none of the phases of a pumping process, the pumped out gas comes into contact with a lubricant, and no volatile components, which are primarily contained in a lubricant, are diffused toward the high vacuum side and pollute it.
Because the wall of a vacuum chamber is always loaded with water, it reaches, via the turbomolecular pump, the suction region and, thus, penetrates into the pump chamber of the diaphragm pump. In particular during heating of the vacuum chamber, an increased amount of water is released. The released water must be pumped into the atmosphere with the forevacuum pump. In case the water vapor pressure during the compression in the first stage of the forevacuum pump is lower, because of the temperature relationships, than the pressure in the intermediate vacuum chamber of the forevacuum pump, the water vapor becomes condensed in front of the outlet valve and evaporates again upon the increase of the pump chamber. As a result, the pressure is inadequate for opening of the outlet valve, and the forevacuum pressure increases to an inadmissible high value. Often, the maximum permissible forevacuum pressure of a turbomolecular pump is exceeded, and its end rotational speed cannot be achieved. As a result, the vacuum in the vacuum chamber remains below the desired value.
The use of gas ballast system, as in the rotary vane pumps, here does not make sense. Diaphragm pumps and piston pumps or comparable apparatuses do not have the particularities of the rotary vane pumps. Specifically, they do not include a gas ballast valve which blocks the admitted gas from reaching the suction region. The gas ballast must directly be admitted into the pump chamber of the first stage which can result in impermissible increase of the end pressure. It is possible to reduce the admittable amount of gas by providing a restriction having a very narrow cross-section. However, in this case, a danger of clogging arises, and the operational reliability is reduced. By using a controlled value, it is possible to prevent the increase of pressure in the pump chamber as a result of admission of the gas ballast. However, this solution noticeably increases the costs of the entire system.
Accordingly, an object of the present invention is to provide a gas ballast system for multi-stage vacuum pumps, e.g., diaphragm pumps or piston pumps.
Another object of the present invention is to provide a gas ballast system for multi-stage vacuum pump with which an adequate amount of gas can be delivered into the pump chamber of the first stage, without adversely affecting the end vacuum and without a necessity to use a restriction with a narrow cross-section.
A further object of the present invention is to provide a gas ballast system for multi-stage vacuum pumps which would be inexpensive and without the use of additional expensive components.